1. Field of the Invention
The present invention relates to a process and an apparatus for treating and recovering an exhaust gas let out from a chemical vapor deposition (CVD) system, more particularly to a process and an apparatus for treating and recovering an exhaust gas let out from CVD system for forming silicon epitaxial films, polycrystalline films or amorphous films using halogenosilane gases in a semiconductor manufacturing process.
2. Description of the Prior Art
The epitaxial (single crystal growth) process, which is a silicon CVD process, is employed for preparation of substrate for field-effect MOS(metal-oxide-semiconductor) transistor or for formation of emitter layers in bipolar transistors.
The epitaxial process is generally carried out by using a halogenosilane gas such as TCS(trichlorosilane) or DCS(dichlorosilane) which is diluted with hydrogen as a raw gas before introduction into a process chamber and by heat-decomposing the raw gas by heating the substrate placed in the process chamber to about 1100° C. to effect deposition of silicon on the substrate.
The above process is generally carried out under a pressure condition of atmospheric to 100 Pa (Pascal).
Meanwhile, the polycrystal growth process is employed for forming gate electrodes of field effect MOS transistor and ground layers for capacitors.
In the polycrystal growth process, a halogenosilane gas such as TCS or DCS is diluted with hydrogen, and the thus diluted gas is introduced into a process chamber in which a substrate heated to about 800° C. is loaded to effect heat decomposition of the raw gas to achieve deposition of silicon on the substrate. This processing is usually carried out under a vacuum condition of about 100 Pa.
Further, in such process, for the purpose of control of moisture to be adsorbed by wafers as they are loaded in and out of the process chamber, a moisture monitor, e.g., Fourier Transform infrared spectrometer (FT-IR) is occasionally attached to an exhaust piping system.
In such crystal growth process as described above, the amount of the raw gas which contributes actually to the deposition of silicon on the substrate is about 5%, and the most of the remining raw gas is exhausted without contribution together with intermediate products (about several % of the total amount) from the chamber.
The exhaust gas let out from the chamber is detoxicated by a detoxicating device which removes the raw gas and the intermediate products, and only hydrogen as the carrier gas and nitrogen as the purge gas are released into the atmosphere.
In the epitaxial process, however, there is a problem that intermediate products (by-products) formed during the process adhere or deposit on the inner wall surface of the exhaust piping to be likely to cause clogging of the exhaust piping with the deposit.
Such intermediate products are compounds of silicon and chlorine or compounds of silicon and hydrogen. These compounds form polymers at room temperatures on the inner wall surface of the exhaust piping.
The formed polymers are converted to highly reactive (self-ignitable or explosive) materials, for example, polysiloxanes, by the moisture contained in the atmosphere.
Accordingly, when the exhaust piping is dissembled to be open to the atmosphere in order to remove the polymers deposited in the inner wall surface of the exhaust piping, various preparations and contrivances are required, being causative of dropping the operation efficiency of the CVD system.
Further, it is practiced to feed an etching gas such as chlorine trifluoride (CIF3) into the exhaust piping in order to prevent deposition of the polymers.
It is true, however, that the intermediate products or the polymers deposited on the inner wall surface of the piping can be removed according to this method, but the method involves a problem in that the exhaust piping itself is corroded by the strong etching property of the etching gas or the etching gas can even cause formation of holes in the piping.
Further, the etching gas such as chlorine trifluoride and the raw gas employed in the epitaxial process cannot usually be treated by the same detoxicating device, so that a plurality of detoxicating units must be used selectively depending on the gas which is fed.
On the other hand, there is a proposed method, in order to prevent intermediate products from adhering or depositing on the exhaust piping, to heat the piping constantly to a temperature of about 150° C.
According to this method, however, if the temperature of the piping is low at some parts, the intermediate products are deposited selectively to such low-temperature parts.
The piping between the detoxicating device and the CVD system usually contains complicated bends from the requirement of reducing the installation area, and it is difficult to heat or heat-insulate the piping uniformly. Actually, the maintenance of the piping has been carried out by dissembling the piping to remove the intermediate products deposited at the low-temperature portions.
While a scrubber employing water is frequently used for detoxification of TCS or DCS, solid silicon dioxide (SiO2) is formed by the reaction between water and TCS or DCS, so that the circulation water employed in the scrubber is provided with means for removing SiO2. However, since the removed SiO2 contains hydrogen, it cannot be exhausted as such. Thus, it has been practiced to carry out treatment of SiO2 by reacting it with hydrogen fluoride (HF). Since these procedures are carried out as periodical maintenance of the detoxicating device, not only the operation rate of the CVD system is lowered, but also chemical agents for removing the SiO2 formed, personnel, etc. cost additionally.